1. Field
The invention relates to a marker for a magnetic theft protection system comprising at least one amorphous ferromagnetic alarm strip and at least one semi-hard magnetic activation strip. These markers can be used in magneto-elastic and in harmonic theft protection systems.
2. Description of Related Art
Certain magnetic theft protection systems and markers are for example, disclosed in EP 0 121 649 B1 and U.S. Pat. No. 5,729,200. In these theft protection systems, a detector system transmits a pulse which excites the alarm strip of the indication element, making the alarm strip vibrate with a characteristic resonant frequency. As a result, the detector system detects the alarm strip and triggers an alarm.
In magneto-elastic systems, the activation strip activates the alarm strip by means of magnetisation. In these systems, the alarm strip vibrates with a characteristic resonant frequency, while the activation strip is magnetised. The alarm strip is deactivated by a change of its resonant frequency. This is achieved by the demagnetisation of the semi-hard magnetic activation strip, making the alarm element vibrate at a different frequency which is not detected by the detector system.
In contrast to magneto-elastic systems, the magnetised activation strip of harmonic theft protection systems is used to deactivate the alarm strip.
The markers of theft protection systems are increasingly installed directly in or on the product to be secured, a method known as source tagging. In these systems, an operator is often responsible for the magnetisation or demagnetisation of the activation strip. This results in the additional requirement that it must be possible to magnetise or demagnetise the semi-hard magnetic alloy of the activation strip from a greater distance or using smaller fields.
It has been found that the coercive force has to be limited to a maximum of 25 A/cm for these applications. On the other hand, an adequate opposing field stability is also required, which determines the lower limit value of coercive force. Only coercive forces from 10 A/cm are suitable for this purpose.
Certain activation strips made of a semi-hard magnetic alloy with a coercive force meeting these requirements are, for example, disclosed in DE 197 32 872 and U.S. Pat. No. 5,685,921.